Automatic transmissions are well known, in particular, in motor vehicle manufacturing.
Hydraulic automatic transmissions constitute a major kind of automatic transmissions of motor vehicles for its smooth running, facility to operate and good power characteristics as well as for its guaranteed security. However, using hydraulic drives lowers their transmission efficiencies and adds to the cost.
Electromechanical automatic transmissions merely employ gear drives, so they have a high efficiency and require a comparatively low cost. However they have problems associated with a combination of gear change and power transmission and there are some errors in their flexible elements. Accordingly, they can not be considered suitable to get a good running.
Stepless automatic transmissions or continuously variable transmissions were regarded as good from the outset and now are in maturity. They overcome such problems with the aid of electronic controlling systems as torque capacity, noise and reliability. However cost problem still remains unsolved.
In dual-path hybrid automatic transmissions consisting of two paths i.e. a planetary drive path and a generator-electromotor drive path, such as the examples disclosed in U.S. Pat. No. 6,416,437 B2, US2003/0176955 A1 and International Publication No. WO242658, and the Toyota hybrid system (THS), the planetary drive has a high efficiency because a direct transfer of power is made by means of gears therein but the generator-electromotor drive is low in efficiency due to transferring power through several energy conversions, and yet the said transmissions become more complex in construction for their complicated driving process, which results in a high overall cost.
In six or seven stage automatic transmissions, speed ratio ranges are narrow in each stage and wide between the highest and lowest stages, and thus a prompter speed change is achievable with the aid of electronic controls. Therefore, fuel consumption is low and respondability is high, which improves the movability and accelerability of motor vehicles. However, the same transmissions become more complex in construction and require a sophisticated control system, which adds to the cost. In addition, using hydraulic converters causes not a small loss in power transfer and a constant control operation forms many inflection points on the characteristic curves of the transmissions. Hence, these transmissions can not be considered to be mechanically perfect ones.
Therefore, problems associated with prior art automatic transmissions include transmitting process, reliability, cost, control, respondability and efficiency etc.